1. Field of the Invention
The present invention relates to a color correction technique for a projector and in particular to a color correction method and device taking into account the ambient light in visual environment and the surface color of a screen or a wall on which the projector projects color images.
2. Description of the Related Art
Recently, the color reproduction performance of a projector has improved rapidly and thereby the demand for highly accurate color reproduction to the extent of CRT monitors is growing more and more. There is commercially available a CRT monitor, which is equipped with a sensor for capturing environment light and performs color correction of color images depending on the captured visual environment information before displaying.
On the other hand, in the case of projectors, the surface color of a projection plane is not always white. Accordingly, it is necessary to adjust while balance and brightness depending on the surface color of the projection plane. An example of a projector equipped with such an adjustment circuit has been disclosed in Japanese Patent No. 2973477. This conventional projector detects the color and brightness of the projection plane and adjusts the white balance and brightness based on the detected color and brightness information.
Further, in the case of projectors, not only the surface color of screen or wall but also ambient light affects the color reproduction of a projector. There is disclosed an environment-adaptive projector which is designed with consideration given to influences of such environment in Japanese Patent Application Unexamined publication No. 2001-320725. More specifically, a color sensor measures color information such as RGB or XYZ values from the projection surface on which a white image is projected. The obtained color information of the white projected image is mapped to the CIELab space of the predetermined standard white. When the coordinate values for the actual white are not on the L axis, a bound vector is calculated as the origin within the ab plane at the point where the ab plane intersects the L axis. Then, an inverse vector of the bound vector is obtained as indicating a complementary color, which is output as a corrected color. In practice, such a color correction is repeatedly performed for plural levels of gray and the results are stored in a gamma-correction lookup table (LUT).
However, the conventional projector described in Japanese Patent No. 2973477 adjusts only the white balance and brightness without consideration of color appearance of color images. Accordingly, the color appearance of the whole image cannot be close to a desired color. Since the human color perception is complicated and the color appearance is frequently affected by the view environment and white color, a simple adjustment of white balance and brightness cannot achieve color appearance correspondence.
The adaptive projector described in the publication No. 2001-320725 cannot always obtain satisfactory color reproduction due to the following reasons.    Even if a complementary color to the white color on a projection plane is projected, its reproduced color on the projection plane is not always a gray or achromatic color depending on the spectral reflectance characteristics of the projection plane.    It cannot be determined whether the projected white subjected to color correction using the complementary color pair is a desired white color on the projection plane. There will be developed a color shift problem unless the actual color reproduction physically matches the color correction using the complementary color pair. According to the conventional adaptive projector, the case of color shift occurrence is not considered at all.    As to colors other than achromatic colors, subjected to the color correction using the complementary color pair, it does not ensure that the color appearance of such color becomes desired color appearance in the case where the projected white is different from a standard white due to influences of the visual environment such as the surface color of the projection plane and the ambient light. The human color perception has a complicated mechanism having chromatic adaptation or color constancy. Therefore, the conventional simple color correction using a complementary color pair cannot achieve color appearance matching under different lighting environments or different reference whites.
As described above, the projected image of a projector is subjected to physical influences of the surface color of the projection plane and the ambient light and our color perception is also subjected to the same influences. In such situations, the conventional color correction using simple white balance adjustment or a complementary color pair cannot achieve color appearance matching between the color appearance of the reproduced image on the projection plane and the desired color appearance.